For clarity and precision, specific terminology used in this specification is defined as follows:
Active Core: The central portion of a nuclear reactor which contains fissile and fertile material and in which the fission chain is sustained and most of the energy of fission is released as heat.
Blanket Region: An active core region immediately surrounding a seed region containing predominantly fertile material and characterized by conversion of the fertile material into fissile material by neutron capture.
Breeder Reactor: A nuclear reactor which produces a fissile material to replace that used to maintain the fission chain. Further limited herein to nuclear reactors which produce more fissile material than they consume.
Breeding Ratio (eta): The ratio of the number of fissile atoms produced to the number of fissile atoms that have been consumed.
Conversion Ratio: The ratio of the instantaneous rate of production of fissile atoms to the instantaneous rate of destruction of fissile atoms.
Doubling Time: The time required for a breeder reactor to produce a surplus amount of fissile material equal to that required for the initial charge of inventory of the reactor, after accounting for reprocessing and refabrication losses.
Epithermal Reactor: A nuclear reactor characterized by a neutron energy spectrum in which more than half of the fissions result from the absorption of neutrons having energies above 0.6 electron (0.6 ev) and a moderating power per fissile atom (.xi..SIGMA./NU) less than 1000.
Fast Energies: A nuclear reactor characterized by a neutron energy spectrum in which more than half of the fissions result from the absorption of neutrons having energies greater than 200,000 electron volts (0.2 Mev) and a moderating power per fissile atom (.xi..SIGMA./NU) less than 10.
Fertile Material: Material which can be converted into fissile material through neutron capture; for example thorium-232 and uranium-238 fertile materials are converted respectively to uranium-233 and plutonium-239 fissile material.
Fissile Material: Material which will undergo fissions with neutrons of all energies; including thermal to fast neutrons; for example uranium-233, uranium-235 and plutonium-239.
Fuel: Designates either fissile or fertile material or a combination of both.
Intermediate Reactor: A nuclear reactor characterized by a neutron spectrum in which more than half of the fissions result from the absorption of neutrons having energies above 3,000 electron volts (3 kev) and a moderating power per fissile atom (.xi..SIGMA./NU) less than 200.
Module: One of a plurality of fuel units comprising an active core region.
Module Geometry: The geometrical configuration of a nuclear reactor having modules dependently nuclearly coupled to form an active core.
Movable Region: An active core fuel region disposed for longitudinal movement, in reference to a stationary fuel region during normal reactor operation.
Seed Region: An active core region containing substantial fissile material and characterized by neutron leakage to a blanket region.
Stationary Region: An active core fuel region which remains fixed during normal reactor operation.
Thermal Reactor: A nuclear reactor characterized by a neutron spectrum in which more than half of the fissions result from the absorption of neutrons having a substantially Maxwellian number-energy distribution about an energy value equal to KT, where K is a constant and T is the reactor temperature in degrees Kelvin and a moderating power per fissile atom (.xi..SIGMA.NU) greater than 1,000. In such a reactor, more than half of the fissions result from the absorption of neutrons having neutron energies below 0.6 electron volts (0.6 ev).
Variable Geometry Control: A means of reactivity control by axially positioning a movable region with respect to a stationary region and thereby changing the leakage of neutrons from the movable region to the stationary region.
U.S. Pat. No. 2,708,656 issued to E. Fermi et al on May 17, 1955, describes physics principles applicable to nuclear reactors. U.S. Pat. No. 2,832,733 issued to L. Szilard on Apr. 29, 1958, describes physics principles applicable to heavy water moderated reactors. Nuclear Reactor Engineering, by Samuel Gladstone and Alexander Sesonske, prepared under the auspices of the Division of Technical Information, U.S. Atomic Energy Commission, Van Nostrand Reinhold Company (1967), describes general terminology used in this specification. The Nuclear Engineering Handbook, edited by H. Etherington, First Edition, McGraw-Hill Book Company (1967) generally describes the mechanical design and operation of reactors.
According to current information, there is believed to be only enough high grade uranium supplies, at present and prospective rates of usage, to last for about 50 years. The Light Water Reactors (LWR), which are the most prevalent type of nuclear reactors in use today, use up only about 1% of the potential energy of their uranium fuel since the uranium-238 isotope, which constitutes 99.3% of natural uranium, is largely unused. A breeder reactor converts the uranium-238 to plutonium, which is fissionable, and thus in principle, permits nearly the entire energy of the uranium to be utilized.
Up to now, it has been considered impossible to perform the breeding process, except possibly at a very low rate, in a light water reactor core. It has, therefore, been necessary to cool the reactor by some other coolant than water. The usual coolants selected have been sodium, steam, or helium. Such reactors are known as fast reactors because the average energy of the neutrons produced is much higher than in light water reactors. Fast breeder reactors are still under development with resulting expenditures totaling many billions of dollars per year worldwide. In order to use these fast reactor cores, new plants will be required, inasmuch as the fast reactor cores under development are not structurally compatible with existing LWR plants.
In the past it has been assumed that with light water it was possible to breed only on the thorium cycle and then only with almost infinitely long doubling times, as in the so-called Light Water Breeder Reactor (LWBR).
Furthermore, extremely high fissile fuel loadings are required for the thorium light water breeder so that very few cores could be started up with available uranium resources. The LWBR demonstration core has two to three times as much fissile fuel per MWe as the LMFBR, and only half the power density necessary to render it a suitable replacement core for standard PWRs. Reprocessing on the thorium cycle, and fabrication of uranium-233 into fuel elements, would be extremely costly and impractical because of the high gamma activity of the irradiated fuel and the difficult thorium chemistry.
In developing breeder reactors it is desirable to achieve short doubling times in order to be able to fuel the increasing number of nuclear reactors which will be required if the world's energy demands continue to increase.
As stated above, it is advantageous for breeders to have short doubling times. Doubling times may be shortened and breeding gains improved by reducing parasitic absorption of neutrons in non-fissile and non-fertile material such as core structural material cladding, moderator, and fission products and also by reducing neutron leakage of neutrons from the reactor core and also by providing a fissile fuel and a neutron energy spectrum wherein a maximum number of neutrons is liberated per average neutron absorption by the fissile fuel; this quantity is known as .eta..
Several different breeder reactor concepts have been proposed n the past. For Instance. In U.S. Pat. No. 4,235,669 to Burgess et al, there is disclosed a nuclear reactor composite fuel assembly for a liquid-cooled breeder which is intended to provide a high breeding ratio and low doubling time. The reference discloses specific structural geometry but does not address the fuel cycle specifically.
U.S. Pat. No. 4,096,033 to Barry discloses a core for a water moderate reactor having tandem arranged fuel regions; an upper region of enriched uranium oxide, a lower region of plutonium oxide and an intermediate region of natural uranium oxide. This invention is directed towards alleviating prior art problems associated with the impact of plutonium on control rod worth and minimizing the effect of the strong plutonium moderator temperature coefficient of reactivity on control requirement. The patent does not teach any method or apparatus For accomplishing high gain breeding in an LWR.
in U.S. Pat. No. 3,998,692 to Sohanan et al, there is disclosed a light water nuclear reactor for breeding U.sup.233. While the reference does disclose the use of U.sup.235, that fuel is associated with U.sup.238 as a dilutant. Also while the reference does disclose the use of Pu.sup.239, it does so with Th.sup.232 as a dilutant. There is no teaching of accomplishing high gain breeding in a LWR on the uranium-plutonium cycle.
In U.S. Patent No. 3,960,655 to Bohanan et al al, there is disclosed another aspect of the nuclear reactor for breeding U.sup.233 discussed above with regard to U.S. Pat. No. 3,998,692.
In U.S. Pat. No. 3,957,575 to Fauth, Jr. et al, there is disclosed the mechanical design for a LWBR using a Th.sup.232 -U.sup.233 fuel system in a seed blanket modular core configuration.
In U.S. Pat. No. 3,859,165 to Radkowsky et al, there is disclosed an epithermal to intermediate spectrum pressurized heavy water breeder reactor.
In U.S. Pat. No. 3,671,392 to Beaudoin et al, there is disclosed a LWBR of the seed blanket type characterized by core modules comprising loosely packed blanket zones enriched with fissile fuel and axial zoning in the seed and blanket regions within each core module.
In U.S. Pat. No. 3,640,844 to Shank et al, there is disclosed a power-flattened seed-blanket reactor core. The flattened power density distribution is achieved with selective loading of fissile material in the reactor core.
In U.S. Pat. No. 3,351,532 to Raab, Jr. et al, there is disclosed a seed-blanket converter-recycle breeder reactor having U.sup.233 fuel, a Th.sup.232 blanket and light water coolant.
None of the references described above provide apparatus for or teach a method for accomplishing high gain breeding in a LWBR on the uranium-plutonium cycle, nor do they teach a method to use existing LWR technology to accomplish the above.